<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>

<body>
    <script>
        var x = 10;
        var obj = {
            x: 20,
            f: function () {
                console.log(this.x); // 20;

                var foo = function () { console.log(this.x); }//10
                foo();
            }
        };

        obj.f();


    </script>
</body>

</html>